netfilter: add missing xt_connlabel.h header in installation

In (c539f017 netfilter: add connlabel conntrack extension), it
was missing the change to the Kbuild file to install the header
in the system.
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/Kbuild

@@ -39,6 +39,7 @@ header-y += xt_bpf.h
 header-y += xt_cluster.h
 header-y += xt_comment.h
 header-y += xt_connbytes.h
+header-y += xt_connlabel.h
 header-y += xt_connlimit.h
 header-y += xt_connmark.h
 header-y += xt_conntrack.h